Wired terrestrial systems offer communications at high data rates, but only while the user is sitting behind a computer. As soon as the user goes to a conference room, walks outside an office building, gets into a car, or drives to a park, the connection is lost. Mobility, however, can be supported in one of two ways, terrestrial-based wireless networks or satellite-based communications systems.
Terrestrial-based wireless networks provide voice or data communications between a mobile user and a fixed user or to other mobile users, as well as communications for modem-equipped computers and other similar devices such as mobile facsimile machines. No existing wireless networks have been optimized for a mix of voice, data, and video, despite the trend towards multimedia traffic. Several wireless and wired standards, such as asynchronous transfer mode (ATM), are being designed to optimize multimedia traffic. Wireless wide area networks (WANs) typically carry voice, whereas wireless local area networks (LANs) typically carry data. Most wireless WAN traffic operates at under 19.2 kbps. Wireless LANs that support data rates up to 10 Mbps have begun to appear, but they are limited in range to tens of meters.
A typical terrestrial-based wireless network includes a grid of service zones or cells, with each cell having a base station situated near its center. A mobile user located in a particular cell is connected to that cell's base station through low-power radio frequency (RF) transmissions. Each base station is connected by trunk lines to other gateways, which in turn are connected by trunk lines to various other networks. Each of these cells require mostly infrastructure development and cover only a very small area. Placing a wireless base station every 200 m to provide global mobile communications is a very costly and time-consuming endeavor. In addition, the elevation angle between the user and the base station is relatively low for terrestrial-based wireless networks. At high frequencies, obstructions such as trees, buildings, signs, etc. can interfere with communications.
To provide wireless service, satellite-based communications systems have been proposed which would provide world-wide coverage. These proposed systems typically include a constellation of satellites in one orbit only, such as geostationary earth orbit (GEO) only or non-geosynchronous orbit (NGSO) only. Communications satellites in geosynchronous orbit provide coverage in predetermined areas on the earth from the equator. Coverage is typically excluded from the oceans so that satellite capacity is not wasted on non-populated waters. Communications satellites in geosynchronous orbit, however, provide limited coverage at higher or lower latitudes than the equator.
Communications satellites in non-geosynchronous orbit, such as medium earth orbit (MEO) or low earth orbit (LEO), travel relative to the Earth's rotation and typically provide high elevation angle coverage at the higher and lower latitudes, and since they are closer to earth, propagation time delays are minimized. Non-geosynchronous communications satellites, however, waste satellite capacity over the oceans during their orbit and currently do not support wideband service to mobile users.
Data rates up to 19.2 kbps, as available from wireless WANs, will not meet future data rate needs of consumers. For example, many computer users are upgrading their wired modems to 56.6 kbps whenever possible. Such users desire a fast response from their modems even while they are away from their desks. In addition, the nature of the information being transferred is changing from short, text-based electronic mail messages to communications with embedded video clips. Such media-rich messages consume high bandwidth and communications resources, thus requiring high data rates to allow them to be transmitted and received within a reasonable period of time.
Thus, there exists a need for a satellite communications system that provides wideband communications to mobile users. There also exists a need for a satellite communications system that provides global communications service while maximizing the useful capacity of the satellites, reducing the perceived time delay, and maximizing the minimum elevation angle across latitudes.